The present invention relates to a drying section of a paper making machine and, more particularly, to a drying section comprised substantially entirely of sequentially arranged, top-felted, single tier dryer sections with a, comparatively, drastically reduced number of dryers within each dryer section to permit compensating for stretching and shrinkage of the paper web.
As used therein, the term "drying section" refers to that part of the paper machine which receives a web of paper emerging from the press section and which extends to the point where the paper web emerges from the drying section about 90 to 99% dry. The paper web proceeds from the drying section to subsequent sections of the machine, e.g. the calender, the coater, after dryer etc. As is well known, a drying section consists of several subsections comprising commonly felted drying cylinders. In the present patent specification, the term "dryer group" or "dryer section" designates a group of commonly felted dryer cylinders, except in a case of a double-tier dryer section where the co-extensive upper and lower dryers, although separately felted, are still considered by the art to be the same "dryer section."
As described in the present Assignee's U.S. Pat. No. 5,311,672, a paper web initially stretches and later shrinks during the course of being dried in the drying section of the paper machine. This is because, despite considerable advances in paper pressing technology, the paper web still emerges from the press section approximately only about 45% dry. The first few drying cylinders merely gradually heat up the paper web to the point where moisture evaporation begins. During this initial stage and while the paper web gives up its "free water", i.e. the water located in the interstitial spaces between the fibers of the paper web, the paper web stretches as it proceeds downstream in the machine direction. Later, the process reverses itself as the drying process releases water molecules lodged within the fibers themselves, and the paper web beings to gradually shrink.
The aforementioned Voith U.S. Pat. No. 5,311,672 patent proposes running with positive draws between the more upstream dryer sections located nearer the wet end of the drying section and running with negative draws, i.e. negative speed differentials, between those dryer sections that are located more downstream, i.e. closer to the very end, i.e. the dry end, of the drying section.
The present inventor has discovered that the prior art does not go far enough to fully compensate for the stretching and shrinking of the paper web. The difficulty stems from the fact that the typical prior art dryer sections are relatively long, containing as they do 6, 7 or even 8 or more commonly felted dryers. These commonly felted dryers run at the same, identical speeds and are traversed by a single felt in the case of single-tier dryer sections and by a pair of felts in the case of a double tier dryer section. Regardless, within any given dryer section, all the dryer surface speeds are practically identical and it is not possible to compensate for stretching and shrinking of the paper web within the dryer sections per se.
In this connection, it is noted that prior to 1990 all drying sections were constructed solely of double-tier dryer sections, each comprising a minimum of 6 dryers or more typically 8 or 10 dryers. This yields an average of approximately at least 7, and in any event, not less than 6 dryers per dryer section.
Since 1990 there has been a concerted switch over from all double-tier drying sections to drying sections comprised of either entirely single-tier dryer sections and/or of a mix of single tier and double-tier dryer sections. To date, most of the installed single tier dryer sections contain at least 5, 6 or even 7, but most typically 6 dryers per dryer section. In any event, the average number of dryers per dryer section is still greater than 5 dryers per section. Reflecting the state of the prior art is U.S. Pat. No. 4,934,067 which shows in FIG. 1 thereof 6 dryer sections each containing 6 dryers for a total of 36 dryers. This patent reflects the design of a machine that has been actually constructed, and scores of similar machines installed around the world. Similarly, the Voith Company's U.S. Pat. No. 5,050,317 shows a plurality of single felted dryer sections in which the number of dryers per dryer section is 8, or even as many as 10 dryers per section. See also, Voith's U.S. Pat. No. 5,177,880.
In drying sections containing a mix of single tier dryer sections and double tier dryer sections, the number of dryers is still on the order of about 6 dryers per dryer section, as reflected in U.S. Pat. No. 5,269,074. This patent again generally reflects the dryer layout of an actual operating drying section.
Although some prior patents have depicted dryer sections with fewer dryers, those patents are illustrative and do not reflect actual feasible machine designs which could dry a paper web from about 43% dry to 90-99% dry as required in an actual paper machine. See e.g. U.S. Pat. No. 4,982,513.
Thus, in present day drying cylinders measuring some 6 to 7 feet in diameter and having intermediate vacuum guiding rolls, a paper web length measuring well over 100 feet within any given dryer section cannot be compensated for the stretching or shrinking thereof within the dryer section. The only points of stretch/shrinkage compensation is at the web transfer zones between dryer sections where, by speeding up or slowing down a down stream dryer section, one is able to pickup the slack i.e. stretch, or compensate for the shrinkage of the paper web. Here too, there are limitations, as it is not desirable to pull a paper web too hard in the early stages of drying, because of the risk of increasing the number of web breakages.